As technological development progresses, modem electronic integrated circuits become increasingly more complex and powerful, they operate at higher speeds, and they consume more electrical power. Along with these developments, the problem of noise within the integrated circuit has become a more significant problem. It is well known in the art that this noise can arise due to parasitic inductances, resistances, and capacitances within the integrated circuit.
Many modem integrated circuits utilize internal clock timing signals to provide a synchronous operation. Examples of high density, high speed clocked logic circuits include microprocessors, microcomputers, math co-processors and the like; of course, other simpler circuits such as memories and smaller scale logic circuits may also be governed by clock timing signals.
Typically, these integrated circuits include a clock driver circuit which is used to provide the internal timing signals throughout the integrated circuit to control its synchronous operation. The internal timing signals coordinate some, if not all, functions within the integrated circuit. The clock driver circuit is, therefore, an important part of the integrated circuit. The following discussion centers upon the use of a clock driver circuit within a microprocessor integrated circuit. However, this discussion is equally applicable to other signal driver circuitry used in all types of integrated circuits, such as address driver circuits, data bus driver circuits, or any other conventional signal driver circuit.
In a microprocessor integrated circuit, the clock driver circuitry is used to drive the internal clock signal to any internal microprocessor circuitry that requires the clock signal to coordinate its operation with other functional elements of the microprocessor. As a result, the clock driver circuit is required to drive the clock signal throughout the microprocessor to numerous internal circuits. The cumulation of the relatively long distances that the signal must travel and the large number of transistors it must drive creates a large load on the output of the clock driver circuit. In order to sufficiently drive the clock signal over the large distances and to the large number of transistors, the clock driver circuit transistors must have sufficient strength to drive the large load. Due to the strong transistors in the driver circuit, the transitions of the driver output from one logic level to another requires large instantaneous current sourcing from the power supply. These large instantaneous currents are often referred to as "switching currents" because they occur when the driver is switching from one state to another. These large instantaneous current flows combined with the resistive, inductive, and capacitive parasitics associated with the power supply lines and the driver load, can create large amounts of noise, often referred to as "switching noise", on the supply lines, the clock signal lines, and other signal lines in the microprocessor.
This noise can cause problems in several areas of a microprocessor. For example, the noise voltage induced in the power supply lines and the signal lines is defined by the equation V=IR + Ldi/dt such that voltage (V) is the summation of the product of the supply or signal current and the parasitic resistance (R) of that line with the product of the parasitic inductance (L) of the supply or signal line and the instantaneous change in the current flowing through that line (di/dt). In modern microprocessors with long signal lines and large switching currents, the noise value can be very significant. This noise may be induced in the power supply lines or signal lines and may cause an error in noise sensitive circuitry within the microprocessor. The large instantaneous currents may also cause a momentary voltage drop on the power supply lines or clock signal lines due to the parasitic resistance of those lines. This momentary voltage drop will slow the device operation for the entire microprocessor. In addition, the large instantaneous currents required by the large clock driver transitions may also cause a power supply drop if the power supply cannot source sufficient current to allow the clock driver circuit to drive its load. This in turn may also slow the device operation throughout the microprocessor. The slowing of the device operation within the microprocessor may in turn cause functional errors due to internal or external race conditions between the functional elements of the microprocessor or its external operating environment.
In order to reduce this noise, capacitors, known as decoupling capacitors, are often used to provide a short term current source or sink for the circuitry in an effort to provide a stable power supply. For example, the decoupling capacitors act as a storage device for electrical charge which can provide a short term current source for the circuitry. When the clock driver circuit transitions from one state to another, the decoupling capacitor will provide a short term current source to supplement the integrated circuit power supply in order to meet the instantaneous currents required by the driver circuit. The local current source provided by the decoupling capacitor reduces the amount of instantaneous current required from the main power supply. This reduces the level of instantaneous current flow through the main power supply lines with the large parasitic inductances and resistances, thus reducing the level of any induced noise voltages or voltage drops.
Decoupling capacitors may also be used to filter out high frequency or low frequency noise from signal lines. It is well known in the art that capacitors with a low capacitance value may be used as a high frequency noise filter, and capacitors with a high capacitance value may be used as a low frequency noise filter.
Typically, decoupling capacitors are placed as close as possible to the circuitry so as to increase their effectiveness. For example, in board level circuit designs, the decoupling capacitors are placed next to the individual integrated circuits on the circuit board. In some instances, decoupling capacitors have been placed within the package containing the integrated circuit. However, the optimum placement of decoupling capacitors is in the integrated circuit itself near the circuitry. While the technology exists to manufacture a parallel-plate capacitor in an integrated circuit, the methodology is expensive because it typically requires the addition of semiconductor fabrication processing steps in order to create the capacitor. This methodology requires the parallel-plate capacitor to be isolated from other circuit elements by layers of insulating material. In addition, fabrication steps are necessary in order to deposit and pattern the conductive and insulative layers of materials forming the capacitor parallel plates and the interlevel dielectric insulator.
It is therefore an object of the present invention to provide a decoupling capacitor formed in an integrated circuit.
It is a further object of the present invention to provide such a decoupling capacitor formed in an integrated circuit which does not require the addition of expensive semiconductor fabrication steps.
It is a further object of the present invention to provide such a decoupling capacitor which is formed in close proximity to the circuitry of the integrated circuit so as to obtain the greatest noise reduction possible.
Still other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having references to the following specification together with its drawings.